In recent years, there has been proposed a technique for providing a wet area device with a radio wave sensor such as a Doppler sensor. The state of a water flow is determined by the Doppler sensor. The operation of the device can be controlled based on the determination result.
For instance, International Patent Publication No. WO 2003/021052 proposes a toilet stool flushing device provided with a Doppler sensor. In this device, the urine flow (water flow) of a user is sensed by analyzing the intensity or frequency of the signal of the Doppler sensor. Thus, an appropriate amount of flush water can be supplied in accordance with the amount of urine.
However, the signal of the Doppler sensor changes with the state of the target and the ambient environment of the Doppler sensor. For instance, in the case where the target is a water flow, a desired signal intensity or frequency may fail to be observed due to the change in the momentum of the water flow or the water pressure, the change in e.g. the distance from the Doppler sensor to the water flow, and the change of the ambient environment such as temperature change. In such cases, the state of the water flow may fail to be determined stably.